P&ID symbology: In ISA S5.1–style instrumentation diagrams, which description best represents the symbol for a “locally mounted instrument” (i.e., field-mounted, not in a control room panel)?

Introduction / Context:
Being fluent in P&ID (Piping and Instrumentation Diagram) symbology is essential for engineers and technicians. ISA S5.1 conventions distinguish where an instrument is mounted (field vs. control room) using standardized shapes and modifiers. This question focuses on recognizing the local (field) instrument symbol.

Given Data / Assumptions:

• ISA-style symbols are used (commonly adopted worldwide with minor variations).
• “Locally mounted” means the device is physically in the field, near the process equipment.
• Tag letters (e.g., PT, TT) appear inside the symbol.

Concept / Approach:
Per ISA conventions: a plain circle with tag letters denotes a field-mounted, locally accessible instrument. A single horizontal line through the circle indicates a control room panel or board-mounted device. Additional shapes (hexagons, squares) or modifiers indicate computer functions, shared displays, or programmable controllers.

Step-by-Step Solution:

Verification / Alternative check:
Referencing ISA S5.1 symbol tables confirms the field instrument as a simple circle without a panel line.

Why Other Options Are Wrong:

• Single-line circle: Indicates control room or panel mounting.
• Hexagon: Computer or programmable functions, not a basic field indicator.
• Diamond/logic: Typically logic or interlock symbols, not a standard instrument bubble.
• Square with cross: Not the ISA symbol for a local instrument.

Common Pitfalls:
Misreading panel-mounted vs. local due to faint or missing lines; always check for the horizontal line across the instrument bubble.

Final Answer:
A plain circle with the tag text inside (no horizontal line through it)